1. Field of the Invention
The invention relates to a process for separating oil that accumulates in alkaline cleaning solutions used to clean metal parts contaminated with oil or similar materials widely used as lubricants in machining and/or as temporary protection against corrosion. Such oily materials normally must be substantially totally removed from metal surfaces in order to prepare them for uniform formation on the surface of a coating that will provide long term protection against corrosion.
The term "alkaline cleaning solutions" as used herein includes all aqueous solutions that contain dissolved alkali or alkaline earth metal hydroxides, carbonates, borates, phosphates, or silicates. The borates, phosphates, and silicates in this class include both simple and condensed types, such as metasilicate, pyrophosphate and tripolyphosphate, and borax and the like. The alkali and alkaline earth metals include sodium, potassium, magnesium, calcium, barium, and the like. Alkaline cleaning solutions of this type can be adapted to specific uses by choosing specific mixtures and ratios of the borate, hydroxide, phosphate, silicate, and/or carbonate constituents. Thus, for certain end uses, phosphates may be preferred to carbonates, silicates may be used exclusive of the other alkaline salts, or, more often, combinations of hydroxides and one of the less strongly alkaline salts, which may be utilized in widely varying ratios to each other, will be preferred.
Many alkaline cleaning solutions of this type, usually also including suitable surfactants and, optionally, sequestering agents (which are usually also chelating agents), are known in the prior art, are commercially available and are widely used for removing oily type soils from metals. Cleaners of this type are very effective when freshly formulated, but when repeatedly reused eventually accumulate so much dispersed and/or emulsified oil that their cleaning performance becomes unsatisfactory. To avoid the expense of replacing the alkaline salts and other constituents, separation of the oil from such used cleaning solutions is desirable.
2. Statement of Related Art
Normally, alkaline cleaner compositions now used for metal surface preparation contain a surfactant component, which may be a single chemical type of surfactant or a mixture of such chemical types, including any or all of the classes of anionic, cationic, amphoteric, and nonionic surfactants. (Cationic surfactants are less commonly used than the other types in metal cleaning formulations, because they are more likely to affect the subsequent processing and treatment of the metal surface in some manner that may be adverse.)
Surfactants and surfactant mixtures that make very stable emulsions of oil in water are very effective for cleaning but present almost insurmountable difficulties when attempting to separate the oil from the emulsion in such a manner as to make it possible to reuse most of the water phase and its cleaning effective contents. Special splitting promoting reagents such as cationic surfactants are commonly added to spent cleaning solutions of this type as part of waste treatment, to avoid discharging oil to water runoff.
Surfactants and surfactant mixtures that are more commonly used in practical metal cleaning operations make semi-stable to moderately stable emulsions. The rapid mechanical motion associated with normal cleaning processes such as spraying aids in dispersing and/or emulsifying oil even into an aqueous phase where it is relatively weakly emulsified. The much more quiescent conditions of a large holding tank or reservoir for the cleaning solution that is commonly provided as part of the cleaning equipment provide an opportunity for weakly emulsified oil to separate from the aqueous cleaner and rise to the surface of the tank, from which it can be removed by skimming and/or controlled overflow of a fraction of the tank contents. However, the spontaneous separation of the oil in this manner, otherwise known as "oil splitting", is usually both slower and less complete than would be desirable, with cleaning solutions containing the presently conventional surfactants. This is especially true when cleaning corrosion protective oils, which often contain materials that can act as emulsifiers or emulsion stabilizers for the oils in an alkaline cleaning solution. One object of this invention is to improve the speed and/or effectiveness of oil splitting from alkaline cleaning solutions, without substantially diminishing their effectiveness in cleaning.
A large field of art that might be considered related, although it actually has significant technical distinctions, is that of breaking emulsions of water in oil, particularly as part of petroleum extraction and recovery. As a consequence of the difference in the nature of the continuous phase of the emulsions, it is doubtful that such art is sufficiently closely related to be of value in attempting to improve oil splitting in emulsions of oil in water. However, as a matter of interest it is noted that such art as U.S. Pat. No. b 4,474,682 of Oct. 2, 1984 to Billenstein et al. teaches the use of ethoxylated and propoxylated polyethylene polyamines as emulsion breakers for water in oil emulsions.